This kind of telecommunications network combines a plurality of equipments, connections, and functions dedicated to transporting data coming from terminal equipments connected to the network. In particular, the transport functions may be implemented by activating routing and transmission protocols. A telecommunications network administered by an operator is also referred to as a domain.
An IP connectivity service provider deploys a dedicated architecture to enable terminal equipment users to be contacted. Access to the IP connectivity service is managed by the service provider using the telecommunications network of an operator to route data packets sent by the terminal equipments to their final destinations. In some circumstances, said service provider is also the telecommunications network operator.
This kind of service provider assigns an IP address, generally a public IP address, to a home gateway placed between a home network and the public network or IP domain of the operator. The home gateway generally assigns private IP addresses to the terminals of its home network.
Below, the expression “home gateway” refers to any equipment for interconnecting a private network and a network operated by a service provider, the private network being either a home network or a business network.
The network operated by the service providers is also referred to below as a public network.
Located on the path of data packets between a terminal of its private home network and the IP domain of the operator, the home gateway includes, as known in the art, a table in which it associates the private IP address and the port associated with that terminal with a public IP address of IPv4 type and, with a port of the same gateway on the public network.
This table is known to the person skilled in the art as a NAT (network address translation) table. There are different types of network address translation e.g. symmetrical, Full Cone, and Port Restricted.
It is commonly accepted in the IP service provider community that IPv4 public addresses are going to run out. To avoid this problem the community has in the past taken action leading to the definition of a new protocol known as the Internet Protocol version 6 (IPv6). This new version of the Internet Protocol provides a large number of IP addresses and a hierarchical routing mechanism offering improved performance. Providers are not indifferent to warnings recently emanating from the Internet Engineering Task Force (IETF), notably in reports presented to the Global Routing Operations Working Group (GROW) concerning the risk of the Internet Assigned Numbers Authority (IRNA) running out of IPv4 addresses by the end of 2010.
However, in practice, this solution of the Ipv6 protocol has not as yet been widely adopted by operators, for financial, strategic, and technical reasons linked to managing the complexity of transition and migration.
To limit the number of IPv4 public addresses necessary to provide an IP connectivity service to an installed base of clients, a solution known as Double NAT or Operator NAT has been proposed and implemented. It entails activating a NAT function within the telecommunications network of the operator so that the home gateways use a private address (instead of a public address) in their outgoing NAT tables. Thus the Operator NAT function translates home gateway private addresses into public addresses, which enables a service provider to economize on a non-negligible number of IPv4 public addresses required to provide the IP connectivity service.
The Operator NAT solution has drawbacks including:                IP data packet processing is made more complex; because of the introduction of a second level of address translation, data packets must be modified twice;        the necessity to adapt the implementation of conventional Application Level Gateway (ALG) signaling protocols such as the Domain Name System (DNS) protocol, the File Transfer Protocol (FTP), and the Session Initiation Protocol (SIP). Taking the SIP, for example, to keep the NAT table of a home gateway up to date, setting up and maintaining a voice over IP session requires frequent exchanges of signaling between the user terminal and the public network via re-registration requests so that the NAT sessions remain active; with Double NAT, such a mechanism must also be provided in the equipment hosting the Operator NAT function; furthermore, the public address and the public port actually being used for the terminal must be communicated to the SIP application;        the public telecommunications network operator offering a degraded IP connectivity service can only be deplored, in particular because functions such as port forwarding and DynDNS are not supported in Operator NAT.        
What is more, such a solution cannot prevent, but can only delay, the phenomenon of IPv4 addresses running out. It is therefore necessary to provide for a changeover to IPv6 in the medium term. Such a changeover will necessarily lead to a period of transition during which IPv6 domains will have to interconnect with IPv4 domains. There is no provision in current networks for facilitating such interconnection efficiently, optimally, and without instantiation of additional states in the network nodes used to provide an IP connectivity service.